I've played a fair bit of Sirlin's Chess2, so I'm going to make a bunch of posts to move the rules onto CV website, in case the game is ever abandoned by Sirlin games.  I will also comment on the game in general.
Sirlin's Chess2 is quite balanced, and has clearly gone through a lot of playtesting.  Being developed by a modern boardgame company owner clearly shows here!

First, Sirlin's Chess2 adds 3 things:

1)win by centerline invasion
2)dueling stones; possible loss of an attacking piece
3)different armies.

Quote from offical rules:

New Win Condition: Midline Invasion
You can still win by checkmate, but you also win if your king crosses the midline of the board. Each move has added significance, because you must weigh how much it helps or hurts each player’s chances of winning by king crossing the midline in addition to the usual considerations of furthering a checkmate.
Just like in Chess 1, it’s illegal to move into check, so to win by Midline Invasion, your King must land on the 5th rank without being in check. Unlike Chess 1 though, there are no stalemates. If you have no legal moves, you lose the game.  While stalemates are common in Chess 1, they aren’t needed in Chess 2 because the Midline Invasion rule provides an even stronger option that a player can aim for when he’s down on material.

In practice, against reasonablely competent players, the majority of games will end by midline invasion.  For one thing, whoever is winning can typically move his king up before he checkmate's his opponent.  The big change, however, is when a player starts to loose, he will usually make a quick attempt at midline invasion win.  This makes the transition between the mid- and end-game very chaotic.
Most non-chess boardgame players will find this a very exciting change; instead of a long slow grind as one player increases his advantage, the the game ends in an explosion of desperate dashes-for-the-midline.  While the player who is in a better position will still usually win, there is more hope for the loosing player.  Having more on the line, it is more exciting for both players, despite the fact that the game still usually ends as expected.
This also esentially eliminates the chess endgame--which most casual players consider the most boring.  Once a player has a significant advantage, chess tends to grind toward an inevitable conclusion.  This is why experienced chess players will conceed when the game gets past a certain point--going through the motions is just a waste of time.
As a side affect, Sirlin's Chess2 games tend to be shorter.  Modern boardgames (not chess variants) tend toward shorter is better, so non-chess enthusasts would generally consider this a good thing.

This is where Sirlin's modern boardgaming design experience is showing....he has designed a change that appeals to the masses (more exciting desperate chance of a win) and eliminated the masses least favorite part of chess (the grinding endgame) and shortened the game in one simple rule.

There is just one problem.
MOST CHESS PLAYERS DON'T LIKE IT.
I don't like it either!!!
Effectively getting rid of checkmate just feels WRONG.

Sum it up=theoretically a good change that appeals to casual players, but chess enthusists won't like it at all.

 

Dueling
Quote from offical rulebook:

Dueling

Dueling allows you to spend a new resource called stones to threaten to destroy a piece that takes one of your pieces. Try to trick the opponent into wasting his stones because if he runs out first, you automatically win any further duels.

You start with 3 stones and gain 1 stone each time you capture an enemy pawn, up to a maximum of 6 stones.

Whenever you would capture any piece, the defender can initiate a duel. If your piece is higher rank than his (ranks: pawn -> knight/bishop -> rook -> queen), he must pay 1 stone to initiate a duel. To duel, you each put 0, 1, or 2 stones in your closed fists, then simultaneously reveal them. All stones revealed are destroyed. The winner of the duel is the one who showed more stones--ties go to the attacker.

If the attacker wins a duel, he takes the piece in question as in normal Chess. If the defender wins, he still loses his piece, but the attacker ALSO loses the piece he attacked with.

Initiating a duel and bidding 0 is a bluff to make the opponent waste stones. The attacker calls your bluff by bidding 0 himself. He wins because attacker always wins on a tie and in addition, the attacker can choose to gain 1 stone or cause the defender to lose 1 stone. (A player can't have more than 6 stones.)

Kings cannot be involved in duels because they have "Diplomatic Immunity." (They can't initiate a duel or be dueled.)

Players with 0 stones cannot initiate duels, but they can be dueled against. When you duel against a player with 0 stones, you must bid 1 and you automatically win the duel. If you lose a pawn in a duel, your opponent does gain a stone.

 

Dueling is another change designed to switch the game up.  Normal chess has a very mathmatical quality to it--good players can predict moves very far in advance.  The farther forward you can think, the bigger your advantage.

Dueling changes this.  Now, sometimes you won't keep a victorious piece.  Consequently, there is only so far out it is practical to predict moves, leveling the playing field a little bit.
Dueling accomplishes this WITHOUT resorting to chance.  The number of stones each player has is public knowledge, and he who correctly reads the importance of the current board position and his opponent will win the duel.  (And the attacker has the advantage, so ties in skill will result in the same board state as if no duel occured.)  However, this requires a very different set of skills than chess.

Consequently, it is possible for someone who is really really good at typical chess to be beaten by a player who is better at reading his opponent and bidding accordingly.  Someone who is bad at bidding may be winning--until they run out of stones.  This gives the othe player a big advantage.

By broadening the useful/necessary skills to win AND lowering the ability to look ahead, a larger variety of player types can be effective players.  Plus each duel is a mini-game, which gives flashes of excitement in the middle of the game.

Again, Sirlin's skill at designing modern boardgames shows.  This is a rule that should appeal to the masses and create some excitement, while lowering the necessity of mapping out future moves.

There is just one problem.
MOST CHESS PLAYERS DON'T LIKE IT.
I don't like it either!!!
Effectively making it uncertain if you are keeping a piece just feels WRONG.

Sum it up=theoretically a good change that appeals to casual players, but chess enthusists won't like it at all.